Suture anchors are often used to attach a length of suture to bone in order to use the suture to secure detached soft tissue to the bone. Suture anchors typically have an anchor body, a suture attachment element, and a bone-engaging member for facilitating placement and retaining the suture anchor within bone. The anchor can be inserted into a preformed hole in the bone, and/or the anchor can be self-tapping and thus can include threads for securing the anchor within bone. Oftentimes suture anchors require the application of torsional forces from an insertion tool at one end of the anchor to drive the suture anchor into bone, as with screw-type anchors. Insertion tools are typically formed from an elongate shank having a mating element formed on a distal end thereof for mating with a corresponding mating element formed on or in the drive head of the fixation device. One common type of driver tool includes a hexagonal-shaped or square-shaped socket for receiving a corresponding hexagonal-shaped or square-shaped head of a suture anchor.
While conventional suture anchors and their drivers are sufficient, they have some drawbacks. Anchor heads with hexagonal or square shaped cross-sections, for example, tend to have a relatively low stripping strength, meaning that under relatively small torque loads the drive head is permanently damaged and torque transfer is thus inhibited. Additionally, this low stripping strength can be further reduced due to the structural integrity of the anchor head, whose drive interface has been compromised or weakened to some degree by the incorporation of a suture attachment element such as an eyelet used to attach the suture to the anchor head. If the head shape of an attachment element decreases the amount of material on the anchor drive head that interfaces with the driver, then the amount of material that needs to yield or be “stripped” from the drive head is reduced, thus reducing the stripping strength of the head.
Conventional suture anchor heads also tend to have a relatively low failure torque, which can result in shearing off of the drive head during insertion. This type of failure is also dependent upon the geometry of the head. In suture anchors, this failure may be exacerbated by the location of the suture attachment element in the head. In particular, if a loop is molded into and embedded within the anchor such that the loop extends outward from the head of the anchor to receive a suture, the entire drive head is relatively weakened and thus has the potential to shear off during insertion.
Suture anchors were historically constructed of implantable metals and alloys which afforded sufficiently high tensile and torsional strength to withstand the rigors of insertion, but the implant remained in the body for prolonged periods of time. Polymer, ceramic, or composite material systems, both biodegradable and non-biodegradable, have been developed for similar applications, but typically have lower tensile and torsional strength than metal counterparts, thus increasing the risk of device failure during application of high torque loads during insertion, as described above. More recently, biodegradable composite material systems have been developed that incorporate filler materials within the polymer matrix, such as calcium phosphate particles, which are osteoconductive. These filled systems may have further reduced tensile or torsional properties compared to unfilled polymer systems. Thus there is a need for an improved torsional drive head for suture anchors that have higher torsional resistance to strippage or shearing off.
One option to increase the failure torque of an anchor is to increase the size of the drive head. Large anchor heads, however, require a large driver tool, which in turn requires a relatively large tunnel to be formed in the bone. This is particularly undesirable, especially where the tunnel is to be formed in the cancellous bone, and where the procedure is minimally invasive and must traverse through a cannula or arthroscope. Accordingly, most suture anchors are adapted for use with a relatively small driver tool, and thus have relatively small drive heads which can result in a low failure torque and a low stripping strength, particularly in harder bone applications. A drive head of improved torsional strength is desirable to reduce the risk of deformation during insertion. Deformation may cause distortion of the anchor near the suture attachment regions, which can inhibit suture slideability necessary to afford knot tying. Additionally, a torsional drive head more resistant to deformation may make a revision procedure easier, as there are some instances where torque driven anchors need to be backed out and perhaps even reinserted.
Accordingly, there remains a need for suture anchors having improved physical properties, and in particular having a high failure torque and a high stripping strength.